1. Field of the Invention
The present invention relates generally to Brillouin Fiber Optic Gyroscopes, and more particularly to an apparatus and method for reducing the Kerr effect and extending the dynamic range of a Brillouin Fiber Optic Gyroscope.
2. Description of the Prior Art
Stimulated Brillouin scattering is a non-linear process that can occur in optical fibers as a parametric interaction among a pump wave, a Brillouin wave and an acoustic wave. When the pump wave and the Brillouin wave propagate in an optical fiber, they generate an acoustic wave through the process of electrostriction, which in turn causes a periodic modulation of the refractive index. This index grating scatters the pump light through Bragg diffraction. The scattered light is shifted in frequency because of the Doppler shift associated with a grating moving at the acoustic velocity. When the Brillouin wave is propagating in the opposite direction to the pump wave, and the frequency of the Brillouin wave is lower than the pump wave by the amount equal to the Doppler shift, the scattered light from the pump wave adds constructively to the Brillouin wave. As a result, the Brillouin wave is amplified while the pump wave is attenuated. The frequency difference and the gain bandwidth of this process are determined by the wavelength of the light and material parameters of the fiber. For a wavelength of 1.3 .mu.m in a silica-based single mode fiber, for example, the frequency difference between the pump wave and the Brillouin wave is about 13 GHz and the gain bandwidth is about 40 MHz.
Such a phenomenon has been utilized to provide for bidirectional laser oscillations in a Brillouin Fiber Optic Gyroscope (BFOG). As described and claimed in U.S. Pat. No. 4,530,097, entitled "Brillouin Ring Laser", assigned to the assignee of the present invention, a BFOG comprises a laser source which provides pump light into a fiber. U.S. Pat. No. 4,530,097 is incorporated herein by reference. A directional coupler splits the pump light traveling into a resonator into two portions, one traveling in the clockwise (CW) direction and the other in the counterclockwise (CCW) direction. The length of the resonator is adjusted so that the pump frequency matches one of the longitudinal modes in the resonator. When the pump power exceeds the threshold level for Brillouin oscillation, Brillouin waves will start propagating, resulting in bidirectional laser oscillations. The CW and CCW Brillouin light waves are combined to produce an interference signal. Once the gyroscope rotates, the resonant frequencies of the CW and CCW Brillouin laser oscillations separate, and the interference signal produces a beat-frequency which is proportional to the rotation rate of the gyroscope.
There are inherent problems with existing BFOG technology that prevents precise measurement of this rotation rate. One such problem is the beat-frequency offset and its non-linear response resulting from the Kerr effect.
The Kerr effect is a phenomenon that occurs when the refractive index of a fiber seen by a Brillouin signal is slightly modified by the signal's own intensity, as well as other light intensities circulating within the resonant cavity of the BFOG. When the circulating intensities of the CW and CCW Brillouin signals are unequal, there is a net imbalance of the optical path length of the cavity seen by these two waves. This imbalance of the Brillouin intensities of the optical path length translates to a beat-frequency offset. In addition, when the imbalance of the Brillouin intensities is not constant as the rotation rate changes, a non-linear scale factor results.
There are two causes for the imbalance of the Brillouin intensities. One is a result of imbalance of the pump intensities themselves. The pump lights are circulating bidirectionally inside the resonator, and the CW pump provides gain for the CCW Brillouin signal while the CCW pump provides gain for the CW Brillouin signal. Any imbalance of the CW and CCW pump intensities thus results in the imbalance of the Brillouin intensities. Existing BFOG technology utilizes a Y-branch beam splitter or an optical coupler to provide an approximate fifty-fifty power split from a single source. Imperfections in manufacture of the splitter or coupler generally result in an imbalance of the Brillouin intensities obtained from such a split. Unequal loss in the fiber arms connecting the Y-branch or the coupler to the fiber ring resonator also results in an imbalance of the Brillouin intensities. This imbalance of Brillouin intensities in turn results in the scale factor offset through the Kerr effect.
The second cause of imbalance is due to the "resonant walk-off effect." When the resonant cavity loop is at rest, both pump waves will have a frequency at a resonant frequency of the cavity. Upon rotation of the loop, each of the counter-propagating pump waves will have a different optical path length around the loop, due to the Sagnac effect. The path length for one of the waves increases, while the path length for the other wave decreases. For instance, when the loop is rotated in a CW direction, the CW-traveling pump wave will have a longer optical path around the loop than the CCW-traveling pump wave. This difference in optical path length causes the resonant frequency for each wave to downshift or upshift accordingly.
In a typical BFOG, the cavity length of the resonator is adjusted through an asymmetrical feedback system so that one of the resonant modes, for example, the resonant mode of the CW pump, coincides with the pump frequency. Thus, when the gyro is not rotating, the resonant mode of the CCW pump light equals the resonant mode of the CW pump light. However, once the gyro rotates, the resonant modes seen by the CW pump light and CCW pump light separate, and the CCW pump light is no longer resonant. This results in a lower CCW pump intensity and accordingly, a lower CW Brillouin intensity. Thus, the imbalance of the Brillouin intensity is a function of rotation rate, and the higher the rotation rate, the larger the imbalance resulting in non-linear scale factor.
The "resonant walk-off effect" also restricts the dynamic range of the gyro rotation rate. As the rotation rate of the gyro increases, the CCW pump intensity decreases to eventually become too low to sustain a CW Brillouin wave. When this happens, the beat signal disappears, and the rotation rate of the gyro cannot be measured.
There are multiple thresholds for different orders of Brillouin lasing in a BFOG. When the pump intensity reaches the first threshold for stimulated Brillouin scattering, the circulating pump power within the resonant cavity is pinned. Any additional pump input power above this pinned level is built up as the first-order Brillouin circulating power. When the first-order Brillouin circulating power reaches the same level as the circulating pump power, which is also the threshold for the second-order Brillouin scattering, the second-order Brillouin circulating wave is generated. The operating window between the first Brillouin threshold and the second Brillouin threshold is referred to as the first operating window of the BFOG.
When the gyroscope is operating at the maximum limit of the first window, i.e., when the input pump power is just below the second-order Brillouin threshold, and when asymmetrical stabilization is employed, the maximum allowed separation of resonator mode frequencies seen by the CW and CCW pump waves in present BFOG technology is .+-.[(.sqroot.3) (.DELTA.f.sub.c /2)], where .DELTA.f.sub.c is the full width at half maximum of the pump cavity resonance. This occurs where the CW pump is stabilized at resonant peak and when the corresponding CCW pump is operating at 0.25 of the CW pump intensity--the minimum to sustain a Brillouin wave.
Since the operating or dynamic range of a BFOG's rotation rate is limited by the "resonant walk off effect," it is desirable to be able to increase the dynamic range of the rotation rate of present BFOGs.
The foregoing problems have been solved, at least in part, by inventions described in U.S. Pat. No. 5,351,252, issued on Sep. 27, 1994, for TECHNIQUE OF REDUCING THE KERR EFFECT AND EXTENDING THE DYNAMIC RANGE IN A BRILLOUIN FIBER OPTIC GYROSCOPE assigned to the assignee of the present application. The copending application is incorporated by reference herein.